Polyolefins having a bimodal molecular weight distribution are desirable because they can combine the advantageous mechanical properties of the high molecular weight fraction with the improved processing properties of the low molecular weight fraction. This provides a polyolefin with a useful and desirable combination of properties, as compared to polyolefins of the high molecular weight fraction or the low molecular weight fraction alone. For example, although typically high molecular weight confers desirable mechanical properties and stable bubble formation onto polyolefin polymers, it also often inhibits extrusion processing by increasing backpressure in extruders, promotes melt fracture defects in the inflating bubble, and potentially, promotes too high a degree of orientation in the finished film. On the other hand, low molecular weight polyolefins typically have excellent processibility, but poor strength. A multimodal molecular weight distribution polyolefin comprising both a low molecular weight fraction and a high molecular weight fraction retaining the desirable mechanical properties, stable bubble formation, reduced extruder backpressure, and inhibited melt fracture is thus desirable. Such polyolefins could find tremendous utility in films and other articles requiring such a useful and desirable combination of properties.
Polyolefins having a multimodal molecular weight distribution may be obtained by physically blending a high molecular weight polyolefin with a low molecular weight polyolefin, as disclosed in U.S. Pat. No. 4,461,873. However, these physically produced blends typically contain high gel levels, which lead to poor film appearance due to those gels. Despite improvements in processability, blending tends to be expensive, requires complete homogeneity of the melt blend, and adds a cumbersome additional blending step to the manufacturing/fabrication process.
Some industrial processes operate using multiple reactor technology to produce a processable bimodal molecular weight distribution polyethylene product in two or more reactors. In a multiple reactor process, each reactor produces a single component of the final product. For example, as described in EP 0 057 420, the production of bimodal molecular weight distribution high density polyethylene is carried out by a two step process, using two reactors in series. In the two step process, the process conditions and the catalyst can be optimized in order to provide a high efficiency and yield for each step in the overall process. However, using multiple reactor technology adds cost and processing considerations.
It is, however, difficult to make bimodal molecular weight distribution polyolefins such as bimodal molecular weight distribution polyethylene, for example, with a single catalyst because two separate sets of reaction conditions are typically needed. Instead, others in the art have tried to produce two polymers together at the same time, in the same reactor, using two different catalysts.
Catalyst systems comprising two different metallocene catalysts are disclosed in the production of bimodal molecular weight distribution polyolefins in EP 0 619 325. EP 0 619 325 describes a process for preparing polyolefins, such as polyethylenes, having a multimodal or at least bimodal molecular weight distribution. The metallocenes used are, for example, a bis(cyclopentadienyl) zirconium dichloride and an ethylene-bis(indenyl) zirconium dichloride. By using the two different metallocene catalysts in the same reactor, a molecular weight distribution is obtained which is at least bimodal.
WO 99/03899 discloses the use of a catalyst composition which produces, in a single reactor, polyethylene with a broad or bimodal molecular weight distribution. The catalyst is prepared from the interaction of silica, previously calcined at 600° C., with dibutylmagnesium, 1-butanol and titanium tetrachloride, and a solution of methylalumoxane and ethylenebis[1-indenyl]zirconium dichloride.
U.S. Pat. No. 7,163,906 discloses a catalyst composition comprising the contact product of at least one metallocene compound, at least one organochromium polymerization catalyst, a fluorided silica, and at least one alkyl aluminum compound, which is then used to polymerize ethylene in an inert atmosphere. The metallocene used in the Examples of U.S. Pat. No. 7,163,906 is bis(n-butylcyclopentadienyl)zirconium dichloride and the organochromium compounds used include dicumene chromium and chromocene. The metallocene-organochromium catalyst system disclosed in U.S. Pat. No. 7,163,906 produced polyethylenes characterized by very broad molecular weight distributions, ranging from 70.3 to 8.4. The polyethylene produced in U.S. Pat. No. 7,163,906 exhibits an intermediate molecular weight distribution with a central high peak attributed to metallocene component and broad tails on both high and low molecular weight sides attributed to the chromium component. Further, U.S. Pat. No. 7,163,906 does not disclose using a molecular switch to activate one catalyst and deactivate the other.
There remain significant challenges in developing processes that can provide control over the various molecular weight fraction of polyolefins with a multimodal molecular weight distribution. Accordingly, there is a need for processes that allow particular control over the composition of each mode of the multimodal molecular weight distribution polymer.